Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsFigure 20 depicts this same system operating with 25 percent outdoor air.The pressure in the zone and return-air path has increased to the point thatthe pressure at the inlet to the air-handling unit (C) is now higher than theambient pressure. This positive pressure differential will force air out throughthe relief damper.Figure 20. VAV system with supply fan only (25% outdoor air)zonereturn-air grille,ceiling plenum,return ductworkAHUAmain supplyductworkVAVterminalunitsupply ductrunouts,diffuserszonestatic pressure relative to outdoors+0-CsupplyfanDBreturn-airdamperHowever, the pressure inside the mixing box (D) must still be lower than theambient pressure in order for air to be drawn in through the outdoor-airdamper. So the return-air damper must be closed far enough to create thenecessary pressure drop (from C to D). This requires the supply fan togenerate a larger pressure differential (inlet to outlet, from B to A) in order todeliver the desired supply airflow and bring in the required amount ofoutdoor air.It also results in a higher pressure in the zone, which could cause doors tostand open without latching. For proper control of building pressure, this“supply fan only” configuration should usually be avoided and either a relieffan or return fan should be used.28 Chilled-Water VAV Systems SYS-APM008-EN